Figure 1. Shear wave splitting measurements in the southern Ryukyu subduction
system showing strong along-arc variability in the fast directions of shear-wave
splitting (a) Blue double arrows display the general trends of the fast directions based
on the work of Long and van der Hilst [2006] and Kuo et al. [2012]. The sawtooth
curve depicts the Ryukyu trench. The grey rectangular frame marks the region of (b).
(b) Map showing topography and shear-wave splitting measured from ocean-bottom
seismometer waveforms in Kuo et al. [2012] for the southernmost Ryukyu subduction
system. The dashed blue line marks the axis of the Okinawa trough. Dashed black
lines show contours of the slab at 100 km and 200 km depths. The colored bars
represent fast directions sampling different tectonic regions, plotted at 50 km depths
of the raypaths: the red and green for the mantle wedge, the blue for the Eurasian
lithosphere, and the orange for the fore-arc mantle. The dashed red line marks the
boundary between the neighboring and the overriding lithospheres in the numerical
models, roughly defined by location of the slab edge.
Figure 2. Three-dimensional model setups. The black lines on the surface mark the
boundary between the EUP and PSP, which may vary among numerical experiments.
The curve with sawtooth marks the Ryukyu trench. The surface of the model is given
free-slip boundary condition except in the grey region where PSP velocity is imposed.
The thick grey box depicts the main calculation domain with higher mesh resolutions.
The dashed grey line on the surface marks the boundary between the neighboring and
the overriding lithospheres in the numerical models. The width of the modeled
segment of the Ryukyu subduction zone in the x-direction is 625.5 km, extending
from ~ 12139.7 to 12755 E.
Figure. 3. (a, b) Cross sections of thermal and flow fields for a model with a thin,
uniform overriding lithosphere, a linear trench, and a slab segment without lateral
variation (model I). The imposed velocity at the surface in the large portion of the
subducting plate is 7 cm yr-1 to the north. (c, d) Cross sections of thermal and flow
fields for a model with a deeper slab and along-strike variation in trench curvature
beneath a thick, uniform lithosphere (model II). The imposed velocity is ~ N 43 W at
~ 6 cm yr-1. The blue lines in (a, c) show the corresponding ISA orientations. Thin
black lines depict the thermal field with 200 C contour intervals. Flow field within
the slab is not shown for clarity in (a, c).
Figure 4. (a, b, c) Cross sections of thermal and flow fields for a model with uniform
back arc opening (model III). The thickness of the overriding plate varies in the
trench-normal direction due to lithospheric stretching. There is no lateral variation in
plate thickness for the overriding lithosphere. The blue rectangular frame in (a) marks
the region of (d). (d) Cross section for the pressure field.
Figure 5. (a, b, c) Cross sections of thermal and flow fields for a model with
substantial variations in plate thickness of the overriding plate in the trench-parallel
direction due to non-uniform lithospheric stretching (model IV). The thickness of the
neighboring lithosphere is ~ 90 km. The blue rectangular frame in (a) marks the
region of (d). (d) Cross section for the pressure field for model IV. (e, f) A horizontal
cross section for the pressure field at 65 km depth and a vertical cross section of
thermal and flow fields for a model with thin neighboring lithosphere (model V).
Figure 6. Thermal and mantle flow fields and the viscosity structure for a reference
models for the southern Ryukyu subduction zone (model VI). (a) Map of flow field at
160 km depth for model VI. Flow field within the slab is not shown for clarity. (b, c)
Vertical cross sections of the thermal and velocity fields for model VI. (d) Vertical
cross section of viscosity structure for model VI. Thin black lines depict the thermal
field with 200 C contour intervals. Note that the trench curvature results in apparent
lateral variations in thickness of the overriding plate in the vertical cross sections (6c).
Figure 7. Maps of flow fields and ISA calculations for model of Figures 6 (model VI).
(a, b, c) Velocity fields (black arrows) and thermal contours (100 C contour intervals)
at various depths (50, 75, and 100 km). Flow field within the slab is not shown for
clarity. (d) Comparison between the observed seismic anisotropy (red and green) and
orientations of ISA (blues). Black lines depict the thermal field at 50 km depth.
Figure 8. (a) Thermal and flow fields at 75 km depth for model VII. The dashed thin
line marks the location for (b). (b) Thermal and flow field for a vertical cross section
(A-A’). (c) Comparison between the observed seismic anisotropy (red and green) and
orientations of ISA (blues). Black lines depict the thermal field at 65 km depth. (d), (e)
Vertical cross sections of the thermal and velocity fields.
Figure 9. Schematic 3D block diagram of the upper mantle for the southern Ryukyu
subduction system. The purple line depicts the particle trajectory of a passive tracer
(model VI) initially located in the shallow mantle of the back arc region. The open
arrow denotes the relative convergent direction of the PSP with respect to the EUP.
The dashed grey lines outline the depth range where the subducting plate (blue lines)
next to the overriding continental lithosphere (thick grey lines). The return flow
around the slab edge predominates at greater depths (green line), whereas complex
flow field develops in the shallow portion of the mantle wedge (pink and thin blue
lines). Note that the green, pink and thin blue lines only show the horizontal
components of the flow field. Black line shows the general pattern of the corner flow.
Figure 10. Maps showing the general trends of the fast directions (blue double arrows)
in the mantle wedge and the interpreted mantle flow direction derived from
geochemical analyses (green and yellow arrows) for the Antilles and the South
Sandwich subduction zones. The sawtooth curve depicts the trench. The gray thick
lines mark the transform boundaries between plates. The open arrow denotes the
relative convergent direction. Triangles show the position of the Holocene volcanoes
(from
the
Smithsonian
Institution,
Global
Volcanism
Program,
http//:www.volcano.si.edu). The gray thin line in (b) marks the boundary between the
South America plate and the Antarctic plate. The black thin lines show the spreading
centers and the transform faults.